The present invention relates generally to crystal growers, and more particularly, to a crystal grower having an integrated Litz coil induction heater.
The process of crystal growing has applications to many industries including the electronics industry. Many semiconductors are formed by the crystal growing process. The process of crystal growing requires heating a base material to a molten state thereby forming a molten pool of crystalline base material. A seed is then placed in contact with the molten pool of material and withdrawn therefrom. As the seed is withdrawn from the molten material pool, a portion of the molten material is withdrawn from the pool in the form of a ribbon thereby growing the crystal.
In order to grow the crystal from the pool of molten material, the pool of molten material must be maintained at a precise temperature depending partly upon the material properties of the base material. Pool temperatures that are below a desired temperature can result in excessive crystal sizes or can result in non-homogenous mixing of the constituent components of the molten material. Temperatures that are above a desired temperature can result in a crystalline structure that is unstable. This instability results in reduced lengths of crystal that can be grown at the increased temperatures. The viscosity of the molten material at increased temperatures cannot support the tension of a crystal being withdrawn from the pool. Therefore, in order to facilitate a desired rate of crystal growth and a desired crystal quality, the pool of molten crystalline base material must be precisely maintained. Induction heaters offer an accurate and easy to control heat source and are commonly used to maintain a desired pool temperature.
Known induction heaters generally consist of a coil of conductive tubing. As a high frequency alternating current is passed through the conductive tubing, a magnetic flux is generated. The coil is positioned generally adjacent an object to be heated and the magnetic flux of the coil induces a current in the material to be heated. Due to the internal resistance of the material to be heated, inducing a current in the material results in the heating of the material. In the case of crystal growing, the induction heater induces enough current to melt the material positioned in a reservoir adjacent the induction heater. Alternatively, the reservoir can be constructed of a material that is responsive to induction heating such that heating of the reservoir results in indirect induction heating of the material. Regardless of which heating strategy is applied, a thermal isolator is generally positioned between the heating target and the induction heater to minimize radiation thermal exchange between the induction coil and the heating target.
Known crystal growers are generally electrically inefficient devices. Due in part to the thermal isolator, a distance is maintained between the heating target and the induction coil. In order to achieve the desired induction current, and therefore temperature, in the target, increased currents must be applied to the induction heater coil. The electrical resistance of the induction heater coil contributes to the total power loss of coil. As the electrical current demand on the induction coil heater increases, the overall efficiency of the induction heater coil decreases. A significant amount of the total energy applied to the induction heater is not used to induce current, and thereby generate heat, in the crystalline material. Therefore, it takes a considerable amount of energy to heat the crystalline base material to a molten state.
In order to induce a sufficient current in the material or the reservoir, the induction heater is subject to a high frequency alternating current. The electrical efficiency of the induction heater is partly determined by the electrical resistance of the induction heater coil to the high frequency alternating current. The distance between the induction heater and the target and the electrical resistance of the induction heater coil contribute to decreases in the overall efficiency of the crystal grower. A user of such a crystal grower must expend considerable resources for all of the energy consumed by the induction heater including that portion of the energy that is not utilized for inducing current in the crystalline base material.
It would therefore be desirable to have a crystal grower capable of heating a crystalline material with appreciable efficiency over known crystal growers. Since the cost of growing crystals is disproportionately weighted toward energy use, such a system would have significant advantages to not only each user, but also to the general environment and economy.